INVESTIGATING THE EXTENT OF THE EARLY MIOCENE COOK CANYON TUFF, A MAJOR PRE-PEACH SPRING TUFF IGNIMBRITE, SW USA, USING GEOCHEMISTRY AND PETROGRAPHY

The Cook Canyon Tuff (CCT) is an 18.9 Ma (Lidzbarski 2014) trachytic ignimbrite exposed in NW Arizona (Buesch & Valentine 1986). It underlies the 18.8 Ma Peach Spring Tuff (PST), a supereruption-scale ignimbrite serving as a stratigraphic marker across extended terrane in SE California, NW Arizona, and S Nevada (Glazner et al 1986; Wells and Hillhouse 1989; Ferguson et al 2013). The eruptive source of the CCT is unknown but may be in the vicinity of Silver Creek caldera (Varga unpub AMS data;), southern Black Mountains, AZ, source of the PST (Ferguson et al 2013). It is exposed in the Kingman area, including in Cook Canyon (Ferguson & Cook 2015), and has been proposed to be regionally extensive (e.g. Varga 2010; Buesch 1993; Gaudio 2003). We compare geochemical and petrographic characteristics of the CCT in the Kingman area (Perry et al 2015; Ferguson & Cook 2015; Gaudio et al 2003; Pratt et al 2014) with potential correlative exposures in NW AZ and CA. We investigated ash flow tuffs at five exposures, all of which underlie PST and have abundant plagioclase and biotite phenocrysts: Hacienda Wash, Sacramento Mtns, CA (HW) and western Warm Springs Wilderness (WS), Union Pass (UP), and two locales W and SW of Yucca, AZ (YN, YS), all in the Black Mtns, AZ.

CCT in the Cook Canyon-Kingman area is nonwelded tuff with ~30% phenocrysts: abundant plagioclase and biotite, rare sanidine, minor hornblende and clinopyroxene, accessory magnetite, ilmenite, apatite, zircon (Perry et al 2015; Ferguson & Cook 2015; Buesch & Valentine 1986; Pratt et al 2014). Whole rock compositions (XRF) range from 63 to 68 wt % SiO₂ (mostly 66-68), with 2.5-4% FeO_t, 5-6% K₂O, 0.7-0.9% TiO₂, 420-590 ppm Sr, 450-620 ppm Zr). Biotite consistently has high and uniform TiO₂(~6 wt%).

Whole rock compositions (XRF) and biotite (SEM-EDS) compositions suggest that YS and YN (WR: 71-74 Wt% SiO₂, 0.3-0.5 TiO₂, 1.8-2.3 FeO_T, 130-300 Sr, 140-240 Zr; Bt TiO₂ ~5 wt%) are not correlative to CCT, but that UP and WS are. HW tuff is moderately to highly altered (indicated by Na₂O <1, K₂O 11-13 wt. %), but Bt retains a composition similar to that of CCT; a CCT correlation is possible but uncertain. This study confirms the wide extent of the CCT, which, when combined with thicknesses up to 60 m, supports the interpretation that it was a large eruption that shortly preceded the supereruptive PST.